Enantioselective Toxicity of Chiral Pesticides in Aquatic Systems

Chapter 7

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Enantioselective Toxicity of Chiral Pesticides in Aquatic Systems Huijun Liu,1 Weidan Cai,1 and Weiping Liu*,2 1School

of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou, People’s Republic of China 2Ministry of Education Key Laboratory of Environmental Remediation and Ecosystem Health, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310058, China *E-mail: [email protected]

Chiral pesticides may differ markedly in their environmental fate, bioavailability and toxicity. A number of reports have shown that chiral pesticides may cause enantioselective toxicities to aquatic organisms. In this chapter, the acute toxic manifestations and long-term effects of chiral pesticides on aquatic organisms, the factors resulting in stereoselective toxicity, and the mechanisms of stereoselective toxicity are reviewed and summarized.

Introduction The use of pesticides in agriculture is growing at a high speed due to the rise in productivity. In general, pesticides are considered as a class of important pollutants that are widespread throughout the world. More than 40% of the existing agrochemicals in China contain chiral centers, and most are produced and used as racemic mixtures (1). However, enantiomers usually differ in their biological properties as a result of their interaction with enzymes or other naturally occurring chiral molecules. Enantioselectivity plays an important role in the environmental fate and ecological risks of a chiral compound because many biologically mediated environmental processes are enantioselective. Many current chiral insecticides have high activity against nontarget organisms © 2011 American Chemical Society In Chiral Pesticides: Stereoselectivity and Its Consequences; Garrison, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


(2). In particular, two classes of insecticides, synthetic pyrethroids (SPs) and organophosphates (OPs), are acutely toxic to a wide range of aquatic organisms at trace levels (3, 4). Contamination of surface aquatic ecosystems by these compounds is a great environmental concern (5, 6). Pesticides may affect estuarine microorganisms via spills, runoff, and drift (7). Metolachlor is regularly detected in North American and European surface waters during the crop growing season (8–10). In Switzerland, peak exposure concentrations have been shown to surpass the suggested chronic water quality criteria for metolachlor, which is set at 0.3 g L-1 (11). Mecoprop, dichlorprop and metolachlor concentrations and enantiomer signatures were determined in Ontario streams, with median concentrations of mecoprop is 44 ng L-1 (0.5-829 ng L-1), dichlorprop is 3.2 ng L-1 (0.5-31 ng L-1) and metolachlor is 94 ng L-1 (24-5160 ng L-1), median EFs for mecoprop and dichlorprop were 0.599 and 0.351, and the median SF for metolachlor was 0.863 in 2006-2007 (12). Though some chiral compounds such as pyrethroids are known to be strongly adsorbed to soil particles, this is not likely to render them immobile post-application because they can be transported in runoff with the soil particles to which they are attached and end up in sediments (13). Once in sediments, they can enter into aquatic ecosystems (14) and be bioavailable to the aquatic food web (15, 16). It has also been reported that runoff of agricultural pesticides into estuaries poses significant toxicological risks to resident organisms (17). But only one of the two enantiomers may be expected to contribute significantly to the aquatic toxicity. Thus, the study of enantioselective toxicity of chiral pesticides to aquatic organisms is important to evaluate the behavior of chiral pesticides and has gained the interest of many researchers. The acute toxic manifestations and long-term effects of chiral pesticides in aquatic organisms, the factors resulting in stereoselective toxicity, and the mechanism of stereoselective toxicity are reviewed in this chapter.

Acute Toxicity The EC50 (LC50 and IC50) is a relatively common index used to evaluate the toxicity of a compound, with a lower value indicating a more toxic potency. Studies have shown that chiral pesticides have a devastating effect on aquatic invertebrates, and dramatic differences between enantiomers have been observed in their acute toxicity to freshwater invertebrates, suggesting that the aquatic toxicity is primarily attributable to a specific enantiomer in the racemate (2). The EC50 or LC50 values of some chiral pesticides reported in the literature for some aquatic species are shown in Table I.

108 In Chiral Pesticides: Stereoselectivity and Its Consequences; Garrison, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Table I. The toxicity of different chiral pesticides to different kinds of water species

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Table I. (Continued). The toxicity of different chiral pesticides to different kinds of water species

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Table I. (Continued). The toxicity of different chiral pesticides to different kinds of water species

Long Term Effects The enantioselective toxicity of chiral pesticides to aquatic organisms can also be shown over time, resulting in long-term effects such as effects on biochemical parameters, structure, and function. Various toxic responses to chiral pesticides have been observed among aquatic organisms. These toxicities can induce biochemical changes in cell components, disturb intracellular signaling pathways, result in blocking mRNA, hamper metabolic protein synthesis, and cause abnormal cell growth/viability. Enantio-difference of biochemical parameters of treated algaes were observed. The Chlorophyll a (Chla) and Chlorophyll b (Chlb) concentrations and the catalase activity of Chlorella pyrenoidosa were used to evaluate the enantioselective toxicity of rac-metolachlor and S-metolachlor (26). In another study, a drastic decrease in algal cell permeability occurred when treated with rac-diclofop and the R(+) isomer, while for S-diclofop, low levels ranging from 1.0 to 5.0 mg L-1 increased algal cell permeability to different extents (19). Daphnia magna, the Japanese medaka Oryzias latipes, and the electric eel Electrophorus electricus were used in a study by Nillos et al. (32) to investigate the enantioselective acetylcholinesterase (AChE) inhibition of the organophosphorus insecticides profenofos, fonofos, and crotoxyphos. The overall results showed variable sensitivity between AChE enzymes from different species as well as variable magnitudes of enantioselectivity in enzyme inhibition. In another study, significantly different endogenous metabolite pattern responses were observed in fish livers when rainbow trout were exposed separately to each of the two enantiomers of triadimefon (33). Chiral pesticides can change cell structure as well. Liu’s study on the toxicity of the chiral herbicides rac-metolachlor and S-metolachlor to C. pyrenoidosa showed ultrastructural morphology differences between control cells and cells grown in the presence of the two herbicides. Exposure to rac- and S-metolachlor resulted in a significant increase in the number and size of starch granules, 111 In Chiral Pesticides: Stereoselectivity and Its Consequences; Garrison, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


while the size of the pyrenoid was reduced. Although the effects of rac- and S-metolachlor on the structure of typical cells had no significant difference, the cells treated with S-metolachlor were lesser in number than the cells treated with rac-metolachlor (26). Xu et al. (34) investigated the differential toxicities of the pyrethroid insecticide lambda-cyhalothrin to zebrafish (Danio rerio). The malformations were induced by the racemate and its (-)-enantiomer at lower concentration (50 μg L-1), whereas the (+)-enantiomer induced malformations at relatively higher concentrations (100 μg L-1). Similarly, the 1R-cis-αS and 1R-trans-αS enantiomers of insecticide beta-cypermethrin showed strong developmental toxicities at concentration of 0.1 mg L-1, while the 1S-cis-αR and 1S-trans-αR induced no malformations at higher concentration (e.g., 0.3 mg L-1) (35). Chiral pesticides may also lead to physiological parameters changes of aquatic organisms. Survival, longevity and fecundity of the freshwater cladoceran Moina macrocopa were reduced at 0.05 mg L-1 and higher concentrations of the insecticide S-methoprene (36). Chiral toxicity of metolachlor in D. magna (29) showed that longevity and the number of broods per female of D. magna were significantly (P < 0.05) affected by rac- and S-metolachlor at different concentrations. Significant inhibitory effects happened when rac-metolachlor concentration was higher than 1.0 mg L-1, but there were no significant affects until S-metolachlor concentration was higher than 10 mg L-1. Based on the significant difference of reproduction parameters (body length, days-to-first-brood, number of broods/young per female) in different treatments at different concentrations, the LOEC and NOEC of Rac-metalaxyl to D. magna were 2 and 1 mg L-1, respectively, whereas those of R-metalaxyl were 1 and 0.1 mg L-1, respectively (31). Significant differences were detected between enantiomers of permethrin in the induction of estrogen-responsive gene expression. At the exposure level of 1,000 ng L-1, the vtg1, esr ∂ and cyp 19b of embryo-larval zebrafish responses to the (-)-trans enantiomer were about 3.2-, 1.8- and 1.5-fold higher, respectively, than those in the group treated with (+)-trans enantiomers (p < 0.05). And (+)-cis increased the RNA level of the cyp 19b gene about 1.5-fold higher than the (-)cis-enantiomer did (37).

Factors That Influence Aquatic Toxicity The direction and degree of enantioselectivity is not always predictable. Both stereochemistry of a chiral compound and environmental conditions influence the direction and rate of enantioselective degradation, which leads to enantioselective toxicity to aquatic organisms. Enantioselectivity in aquatic toxicity of chiral pesticides may closely depend on bioenvironmental conditions (species and biological and physiological conditions), the nature of the chiral pesticides, time, and even nonbiological environmental factors. As a result of different levels of microbial metabolism, degradation, and biological enrichment in aquatic organisms, chiral pesticides may show different acute and chronic toxicity to aquatic organisms. 112 In Chiral Pesticides: Stereoselectivity and Its Consequences; Garrison, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Bioenvironmental Factors Pesticides can be selectively metabolized by organisms. Enantiomers of chiral pesticides, unlike diastereomers, have identical physical–chemical (i.e., achiral) properties (38) and only show differences in selectivity when in a chiral environment. Thus, the relative abundance of enantiomers is subject to change after metabolic processes due to numerous enzymes and receptors having symmetry (i.e., chiral) dependence (24). Aquatic organisms at different physiological stages show sensitivity to chiral pesticides stereoselectively. Days to first brood of D. magna was not affected by the presence of either rac- and S-metolachlor. But after D. magna gave birth to the first brood, the difference in toxicity to D. magna between rac- and S-metolachlor became significant (29). Microbially-mediated processes in aquatic environments can include enantioselective degradation and can therefore change the enantiomeric composition over time. In one study, o,p-DDD, a chiral metabolite, was found in fish tissue after exposure to DDT, occurring primarily as the (S)-(–)-enantiomer. The enantiomer fraction for o,p-DDD in two-thirds of the fish samples was between 0.29 and 0.44 (39), indicating that biotransformation had occurred. In addition, studies about metabolism and dissipation of fipronil (40) indicated that it may be converted to desulfinyl, sulfone, and sulfide metabolites, which compared to parent fipronil are reported to have equal or more toxic potential and are more environmentally stable in water and soil. The fipronil was rapidly biotransformed by the rainbow trout with a preference for the biotransformation of the S-(+)-enantiomer to fipronil sulfone (41). It has also been reported that many ALS-inhibiting herbicides and their derivatives are chiral with one or more enantiomers, such as all the imidazolinone herbicides and some sulfonylurea and sulfonylamino-carbonyltriazolinone derivatives (42). Enantioselective biodegradation of the enantiomers was observed in both field applications and laboratory microcosms (43). The isomers of pyrethroids were generally degraded selectively. The EF values for cis-permethrin obtained ranged from 0.412 to 0.535 in all samples, indicating that there was enantioselective degradation (44). Kurt-Karakus et al. compared concentrations and stereoisomer ratios of dichlorprop in Ontario streams in 2006–2007 vs. 2003–2004 (12). The results showed that residues were dominated by the S(−) enantiomer in both time periods (83–90%). The presence of substantial S(−)-mecoprop and S(−)dichlorprop residues in all years is likely an indication of enantioselective degradation and/or interconversion of enantiomers within the watershed and / or streams. The enantioselective degradation of α-hexachlorocyclohexane (α-HCH) in the water and snow samples collected at Amituk Lake on Cornwallis Island was found, with ERs were 0.77 ± 0.004 (45). Selective degradation of chiral pesticides in the environment lead to difference in their retention time, which may result in different chronic toxicity to the aquatics. Konwick et al. studied bioaccumulation and biotransformation of chiral triazole fungicides in rainbow trout (Oncorhynchus mykiss) (24). The results showed that half-lives of triazoles ranged from 1.0 ± 0.2 days for tebuconazole to 2.5 ± 0.6 days for penconazole in rainbow trout. The rapid formation of fipronil sulfone, a known metabolite of fipronil, was found to persist 113 In Chiral Pesticides: Stereoselectivity and Its Consequences; Garrison, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


longer (t1/2~2 d) than its parent compound fipronil (t1/2~0.6 d) and needs to be considered in fate studies of fipronil (41). Pereira and Hostettler (46) found no evidence of degradation of deethylatrazine, a major metabolite of atrazine, during its transit time (45–65 d) in the entire navigable reach of the Mississippi River, U.S.A. And a laboratory study found this degradation product to be nearly as toxic as the parent compound to estuarine microbial communities (47). Moreover, in some cases, it has been demonstrated that the less active enantiomer persists in the environment for much more time than the enantiomer with more insecticidal activity (48). More reports showed that the effects of chiral pesticides on aquatic environments are also due to their enantioselective degradation products, which can be more toxic than the original substances (40). Pesticides can also be bioaccumulated by organisms and then the toxicity of chiral pesticides may change. Zhao et al. (49) reported that the enantioselectivity in chronic aquatic toxicity is reasonably consistent with the accumulation of cis-BF by organism. As the degradation of chiral pesticides may be enantioselective, resulting in different distribution patterns and bioaccumulation potentials between enantiomers (50). A preferential degradation of the (−)-enantiomer of cis-bifenthrin resulted in a relative enrichment of the aquatically active (+)-enantiomer (2). Some pesticides like endosulfan which was not toxic to algae at levels likely to be found in the environment (< 1 mg L-1), the compound may bioaccumulate in algae and be consumed in higher concentrations by grazers (7), such pesticide may pose a threat to higher estuarine organisms. Rao and Lal (51) found rapid uptake of the commonly used agricultural pesticide endosulfan in the cyanobacteria Aulosira and Anabaena, with levels reaching 700 times the exposed dose within 48 h. Pesticides accumulated in prey species may then cause toxicity in the organisms consuming them or continue to bioconcentrate through the food web (7). Enrichments in organic nutrients shifted the enantioselectivity for methyl-dichlorprop towards the preferential degradation of the (S)-enantiomer (52). Chiral pesticides may be sediment adsorption, and contaminated sediments can be continually resuspended, especially true in estuaries, because of tidal action and dredging. Hence, the degradation of chiral pesticides in sediments can be the potential element leading to enantioselective toxicity to aquatic organisms. S-(+)-lactofen or S(+)-desethyl lactofen was preferentially degraded in sediments, resulting in relative enrichment of the R-(-)-form (25). Enantioselective attenuation of cis-BF and cis-PM was performed in the sediments collected at a site next to a nursery in Southern California (43). The results showed that the S-enantiomers of BF and PM were preferentially degraded, and then cause relative enrichment of the R-enantiomers. Enantioselectivity was also evaluated in the course of degradation of cypermethrin by Ceriodaphnia dubia in sediments (53). Nonbiological Environmental Factors Although nonbiological environmental factors will not change their aquatic toxicity directly, the biological growth and metabolism of the aquatic organisms can be affected by these nonbiological conditions, which may indirectly lead to stereoselectivity in toxicity. Factors such as pH, salinity and nutrient 114 In Chiral Pesticides: Stereoselectivity and Its Consequences; Garrison, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


concentrations in the system may alter the response of aquatic organisms to pesticides. There is evidence that atrazine becomes more toxic to phytoplankton under nutrient-enriched conditions (47). Another study found that natural periphyton communities in low-nutrient environments were less likely to recover from herbicide stress than those in medium- and high-nutrient environments (54). Furthermore, the direction of the degradation can be influenced by the sampling location and environmental conditions (16). These differences in degradation may due to the variations of the microbial population as a result of pH and soil oxidation state (55). The enantioselective behaviors of chiral compounds in the environment might be shifted when interactions with other chiral receptors. Study showed that chitosan can change the enantioselective bioavailability of dichlorprop (56). In the absence of chitosan, the toxicity of (R)-enantiomer to Chlorella pyrenoidosa was more potent than that of the (S)-enantiomer. While (R)-enantiomer was less toxic than (S)-enantiomer in the presence of chitosan. Results showed that the addition of a certain amount of β-CD and HP-β-CD reduced the toxicity of fenoxaprop (FA) to Scenedesmus obliquus and changed the enantioselectivity in toxicity of FA to S. obliquus (22). Analysis of the joint toxicity of binary mixtures of isocarbophos enantiomers toward D. magna clearly showed an additive effect from the individual enantiomers (1), implying that the joint acute toxicity for enantiomer mixtures of chiral xenobiotics may show different modes of interactions when present in a mixture. The enantioselective toxicity to aquatic organisms may vary with different chiral pesticides. Different chiral pesticides (different structure, molecular weight, chiral center, etc.) may show stereoselective toxicity to aquatic organisms. It has been reported that triazoles (myclobutanil and penconazole) with the highest percentage of elimination through biotransformation had either an oxygen atom or hydroxyl group adjacent to a chiral center, while lowest percentage of their elimination through biotransformation did not contain an oxygen atom within the molecule (24). The oxygen atom has great reaction potential through phase one (oxidation, reduction, and hydrolysis) biotransformation reactions, which may result in ring cleavage adjacent to the oxygen atom and/or the formation of hydroxyl groups. Some chiral pesticides containing several chiral centers have multiple pairs of enantiomers, although the active ingredients may show the same effects to the targets, the toxicity may be stereoselective. Many other examples showed stereoselective toxicity of chiral pesticides to the same aquatic (Table I). The toxicity of chiral pesticides to aquatic organisms changes with the concentration. Based on the significant differences in reproduction parameters of D. magna exposed to different treatments at different concentrations, the LOEC and NOEC of rac-metolachlor were 0.01 and 0.001 mg L-1, respectively, while those of S-metolachlor were 0.5 and 0.1 mg L-1, respectively (29). The researches showed that body length, number of broods per female and number of young per female were affected greatly at higher concentration ( 0.01, 0.1, 1, 2, 5 mg L-1) of Rac- and R-metalaxyl. And the significant differences between them were showed by R-metalaxyl at >1.0mg L-1, by Rac-metalaxyl at ≥ 2.0 mg L-1 (31). Compared to blank control, rac-metalaxyl at low level (0.05 and 0.5 mg L-1) stimulated the 115 In Chiral Pesticides: Stereoselectivity and Its Consequences; Garrison, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


growth of S. quadricanda. However, for R-metalaxyl it did not influence algae growth at low concentrations (0.05 mg L-1 and 0.1 mg L-1) (57). Toxicity of chiral pesticides may vary with the time. Significant differences in the LC50 were observed for the two pyrethroid isomers with the (−)-enantiomer 60 times more toxic after 24 h and 162 times more toxic after 96 h than the (+)-λcyhalothrin (18). The order of EC50 (racemate of FV to zebrafish) values were 28.8 g L-1, 17.9 g L-1, 12.4 g L-1, and 8.29 g L-1 at 24 h, 48 h, 72 h and 96 h, (Table I), suggesting that the toxicity of rac-FV to zebrafish increased with time (23). And toxicity of each isomer of FV (aS-2S-FV, aR-2S-FV, aS-2R-FV and aR-2R-FV) showed the same trends. Lin et al. (58) investigated stereoselective acute aquatic toxicity of fosthiazate to D. magna. The differences among isomers varied with the exposure time. The maximum differences observed after exposure of 24, 48, 72 and 96 h were 2.2-fold (pk1/racemate), 3.1-fold (pk1/pk3), 3.1-fold (pk1/pk3), and 2.5-fold (pk1/pk3), respectively. In addition, testing conditions also play a role in toxicity of chiral pesticides. Study showed that the growth of S. capricornutum exposed to the allelochemical ethyl-2-methyl acetoacetate (EMA) under different initial algal densities exhibited a contradictive response. The algal growth was inhibited by EMA at low initial algae densities but stimulated at high initial algae densities (59). The sensitivity of S. vacuolatus to S-metolachlor in synchronous culturing was more than 15 times greater than culturing in continuous light (27). Under aerobic or slightly reduced conditions, biodegradation of fipronil in sediments was essentially nonstereoselective, with the enantiomeric fraction (EF) similar to racemic (EF = 0.5) after 168 d of incubation. However, EF decreased to as low as less than 0.1 following short incubations under anaerobic conditions, suggesting preferential degradation of S-(+)-fipronil in strongly reduced sediments (60). Similar phenomenon of fipronil transformation (to fipronil sulfide) under anaerobic conditions were observed in studies by Jones et al. (61).

Mechanisms of Toxicity Mechanisms of toxicity vary depending on the type of pesticide and the aquatic species exposed. In microorganisms, pesticides have been shown to interfere with respiration, photosynthesis, and biosynthetic reactions as well as cell growth, division, and molecular composition (7). Research showed that paraquat increased the activities of the antioxidant enzymes superoxide dismutase, peroxidase, and catalase compared to the control (62). The higher concentration of paraquat, the higher levels of these antioxidant enzymes. Paraquat reduced the transcript abundance of psaB and rbcL to 7.09 and 29.83% of the control, respectively. Another study of ATZ toxicity to zebrafish (D. rerio) showed the induction of oxidative stress and the alteration of gene expression in liver and ovary samples from female D. rerio (63). Yao et al. (57) reported that sometimes difference in toxicity for individual forms of chiral pesticides originated from different reaction of individual enantiomer with target enzyme. 116 In Chiral Pesticides: Stereoselectivity and Its Consequences; Garrison, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Significant differences were detected between the enantiomers in induction of hepatic gene transcription of male adult zebrafish. At exposure level of 500 ng L-1, the response to the (−)-trans enantiomer was 2.6 and 1.8 times greater than the (+)-trans enantiomer based on zebrafish vtg1 and vtg2 mRNA induction (p < 0.05), respectively (64). It has also been reported that the expressions of estrogen receptor alpha in zebrafish embryo exposed to insecticide acetofenate varied. Data of qRT-PCR showed that there was about 3.2-fold induction in the mRNA levels of ER∂ between fish exposed to (+)-acetofenate and (-)-acetofenate (18). In addition, there was a drastic difference between 1S-cis-bifenthrin and 1R-cis- bifenthrin in their relative estrogenic potential for inducing vitellogenin production of the adult male medaka. The average level of vitellogenin in the adult male medaka exposed to 1S-cis-bifenthrin for 10 d was 1532 ng mg-1, while that with exposure to 1Rcis-bifenthrin was only 12.45 ng mg-1 (65). Cellular systems scavenge the active oxygen species by invoking antioxidative machinery such as superoxide dismutase, catalase, and peroxidase (66). It is well known that oxygenation is the first step in the metabolism of pesticides and other organic xenobiotics. In various detoxification reactions, a number of enzymes use glutathione (GSH) and thioredoxins (TRXs1–3) as cofactors, whereas other ROS-removing enzymes, such as superoxide dismutase, glutathione peroxidase, catalase, thioredoxin peroxides and cytochrome c peroxidase, use alternative hydrogen donors like O, -OH, O2− , and H2O2 (67). So, if an antioxidant enzyme in vivo is inhibited by chiral pesticides stereoselectively, it could lead to different stress responses by the organism to each enantiomer of a chiral pesticide. In chiral notation, R and S refer to the absolute configuration or the orientation in space of the groups around the chiral center of the enantiomer. This orientation is the important factor in determining the fit with enzymes and other biological molecules (33). Thus, the highly stereospecific interaction between an herbicide and an enzyme could be explained by the three-point model proposed by Easson and Stedman in 1993 (68) and the four-location model developed by Mesecarin in 2000 (69). As the isomers differ in their three-dimensional configurations, exists in two forms which are mirror images of each other, and the enzyme is also chiral with a specific stereostructure (70), therefore, there are greater chances that one isomer of a chiral herbicide could block the channel better than the other, causing different enzyme activity. Based on the structural information about the binding site and the inhibitory mechanism, Wang et al. (21) used molecular modeling to explain the enantioselective toxicity between R-(+)- fenamiphos and S-(-)- fenamiphos.

Conclusion Chiral pesticides can show very different biological activity when exposed to an identical biological environment. The factors resulting in stereoselective toxicity to aquatic organisms involve: biological action (biodegradation, metabolism and bioaccumulation, etc.), abiotic environmental factors (indirect factors), type and dose of chiral pesticides, time and testing conditions as well. Although deleterious effects have been documented in aquatic organisms for 117 In Chiral Pesticides: Stereoselectivity and Its Consequences; Garrison, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

many chiral pesticides at different levels, the mechanism of toxicity to aquatic organisms remains unclear, especially at the level of stereoselectivity. Aquatic systems may be contaminated with a number of different pesticides, further study is needed to better understand the joint toxicity of chiral pesticide in aquatic.

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